The present invention relates to a flowmeter that measures a flow rate of air and gas, and a flowmeter system using the flowmeter.
As an airflow meter to measure an intake airflow rate, which is installed on an electronic controlled fuel injection system for an internal combustion engine of an automobile or the like, the thermal-type airflow meter has been widely used because it is able to directly detect an air mass flow. In this case, a heating resistor is made up by winding a platinum wire on a bobbin and coating it with glass, or by forming a thin film resistor on a ceramic substrate or on a silicon substrate, or the like. As a method of detecting a flow rate, the following can be cited. One system is to heat a heating resistor to a constant temperature, and to directly detect a flowing current when there occurs a flow. Another one is to place temperature detecting resistors on both sides of the heating resistor, and to detect a flow rate by a temperature difference between the temperature detecting resistors.
In case of a pulsating flow that involves high pulsating amplitude of an intake airflow rate and partly accompanies a reverse flow, as in case of a low speed of rotation of less than four cylinder engine and a heavy load being applied, the conventional airflow rate measuring device lacks in accuracy; and there is a well known flowmeter disclosed in Japanese Patent Laid-open No. 62-821.
In order to compensate a dynamic pulsation characteristic including a reverse flow, there are methods for an engine control unit and so forth to modify a characteristic curve in a reverse flow region by using a reverse flow discriminating sensor, which are disclosed in Japanese Patent publication No. 8-511627, and the Japanese Patent Laid-open No. 9-15013.
As a method of reducing errors in pulsations, the following are disclosed in Japanese Patent Laid-open No. 11-316145 and No. 11-337382 and so forth, in which an engine control unit or a sensor controlling microcomputer converts a non-linear sensor output signal into a linear signal (flow rate) by using a map or the like to convert it into an airflow rate, and reduces a pulsating amplitude by a filter, and thereafter applies non-linearization processing to the signal again, thus decreasing the amplitude of the final output signal to thereby reduce the errors.
Japanese Patent Laid-open No. 6-265565 and so forth disclose a construction, in which a sensor output of a current meter used for a measuring device is converted into a digital value by an A/D converter, the characteristic is adjusted by means of the functional expression of a memory that records specified coefficients, and the results are put on the display, or the sensor output is converted into an analog value by a D/A converter to output in voltage. With the same construction, a method of reducing influences of fluctuating flow velocity due to pressure fluctuations and so forth is disclosed in the Japanese Published Unexamined Patent Application No. 8-94406, in which the sensor output is converted into a digital value by an A/D converter, and after being linearized, the average is calculated, whereby a flow rate without error can be presented according to the document. These are to be used for making up a closed system as a flow rate measuring device.